Superconducting quantum interference devices (SQUIDs) are the most sensitive magnetic flux detectors currently available. They have extensive versatility in being able to measure any physical quantity that can be converted to a magnetic flux, as for example a magnetic field, a magnetic field gradient, current, voltage, displacement and magnetic susceptibility. The applications for SQUIDs are quite extensive. The SQUID susceptometer has been widely used by scientists in laboratory applications for many years, but more recent advances in SQUID technology have greatly expanded its use beyond the laboratory. In the medical arena, SQUIDs are more commonly being used in magnetoencephalography and magnetocardiology, wherein SQUID magnetometers are used to measure the tiny magnetic signals generated in the brain and heart respectively. SQUIDs have also been used with superconducting magnets to create magneto-ferritometers for monitoring iron levels in the liver. In the commercial marketplace, the military has expended considerable development effort on a variety of SQUID applications. SQUIDs have also found many applications, in geophysics, from prospecting for oil and minerals to earthquake prediction through the use of active and passive SQUID systems. SQUID technology is also useful for non-destructive evaluation, including both the integrity evaluation of structures and the location of submerged or buried structures or members.
SQUIDs combine the physical phenomena of flux quantization and the Josephson effect. Flux quantization means that the flux (.PHI.) through a closed superconducting loop is quantized in units of the flux quantum (.PHI..sub.o .congruent.h/2e.congruent.2.07.times.10.sup.-5 Wb)). The Josephson effect occurs at "boundaries" between superconducting regions of superconductor structures. Such boundaries may be achieved in a number of known ways. "Grain boundary" junctions occur at crystalline grain boundaries and are further discussed in Char et al., U.S. Pat. No. 5,157,466. Boundaries of the "tunneling type" occur where a very thin insulator is interposed between two superconductors. SNS junctions use a very thin normal conductor or weakly superconducting material as the boundary. Nanobridge junctions use a severely restricted area of superconductor to form a weak link, and are further discussed in Kapitulnik, U.S. Pat. No. 5,219,826.
In general, a SQUID comprises a superconducting loop that is broken in at least one place by a Josephson junction. There are two kinds of SQUIDs. The first, the rf SQUID, uses a single Josephson junction to interrupt the current flow around a superconducting loop, and is operated with a radiofrequency flux bias. The second, is the dc SQUID, which has two or more Josephson junctions interrupting a superconducting loop and is biased with a steady or "DC" current bias. Recently, thin film technology has been applied to SQUID construction, making the design of dc SQUIDs more commercially practical. Both the rf and dc SQUID concepts are readily understood by those skilled in the art, and will not be detailed herein. Those not as knowledgeable with SQUID technology and theory of operation are referred to "SQUIDs: Theory & Practice," John Clarke, New Superconductivity Electronics, Kluwer Academic Publishers, 1993, incorporated herein by reference.
In simple terms, a dc SQUID is a magnetic flux-to-voltage convertor, since it provides an output voltage across the Josephson junctions which varies as a function of the total magnetic flux applied to its superconducting loop. The output voltage is periodic in the applied flux, with a period of one flux quantum. By applying a dc bias current and dc flux to the SQUID, to set a quiescent voltage output level, magnetic fields producing a flux in the SQUID of much less than a single flux quantum (.PHI..sub.o) can be detected by measuring the deviation of the SQUID output voltage from the quiescent value. The dc SQUID is the most sensitive detector of magnetic flux available, and displays an enormous frequency response extending from dc to several GHz. The design of a SQUID determines the intrinsic performance of the device. It has been shown that the energy resolution of a low-inductance SQUID operating at 4.2K can approach the quantum limit. Besides the "rf" and "DC" SQUID distinctions, the physical nature and the resultant performance characteristics of a SQUID are related to the type of superconductor material(s) used in its fabrication. Historically, the early superconductors and resultant SQUIDs made therewith exclusively used "low-temperature" superconductor materials that display superconductivity near absolute zero. The use of low-temperature superconductor materials requires them to be placed in liquid helium at 4.2K (-269.degree. C.). SQUIDs and magnetometers and other devices made therefrom are now commonly fabricated using single or multiple-layer thin film depositions and photolithographic and etching techniques well-known in the semiconductor industry. Low-temperature SQUID devices typically use Josephson junctions formed from stacked horizontal superconductor films such as niobium, which are aligned parallel to the substrate.
More recent discovery of higher temperature superconductive materials which display superconductive properties at temperatures over 77K, have increased the accessibility of SQUID technology. The higher temperature superconductors allow for operation in liquid nitrogen (at 77K), rather than in the much more expensive liquid helium. The higher temperature materials are generally referred to as high transition temperature "high-T.sub.c " superconductors or simply "HTS" materials. Ceramics are the most common HTS superconductor materials used in SQUID technology, with the most popular being YBa.sub.2 Cu.sub.3 O.sub.7-x (most commonly called YBCO), which has a transition temperature of approximately 90K. HTS SQUID devices typically use a vertical Josephson junction technology, where the plane of the junction is perpendicular to the substrate, as for example junctions produced by grain-boundaries between two contiguous regions of a superconductor material having different grain or crystal orientations on either side of their juncture. While the present invention will be described with respect to its implementation in a low-temperature superconductor dc SQUID configuration, the principles of this invention apply equally well to SQUIDs employing HTS materials. In order to simplify explanation of the invention, the following discussions and descriptions will be made with reference only to low-temperature superconductors and dc SQUID configurations made therefrom. It will be understood, however, that the invention is not to be limited thereby.
To ensure low-noise operation, a "bare" dc SQUID typically is a low inductance device. A bare SQUID has a low effective flux capture area, resulting in a magnetic field resolution that is insufficient for many applications. Therefore, in practical SQUID magnetometer applications, the SQUID is often coupled to an input circuit generally having one or more pick-up loops or coils of superconductive material capable of capturing much more flux than the relatively small SQUID loop, therefore significantly increasing the magnetic field resolution of the device. Signals from the pick-up loop(s) can be either "directly" or "inductively" coupled to the SQUID loop. In low-temperature SQUID configurations excellent inductive coupling can be implemented by means of a multi-layer transformer configuration using thin film lithographic techniques. In such devices, the thin film superconductor SQUID inductance material is typically formed in the shape of a washer and is covered by an insulating layer on top of which is grown a thin film spiral superconductor coil with as many as several tens of turns, which acts as an input or transformer coil to inductively couple or transfer signals from the input coil to the underlying SQUID inductance loop. The input coil in turn is physically connected to an appropriate external magnetic flux pick-up loop.
The thin film "washer" design implementation achieves low inductance in the SQUID loop and tight coupling to multi-turn input coils by making the loop into a slotted groundplane. A second, thin film modulation coil is usually integrated on top of the SQUID washer as well, in order to couple a flux modulation signal to the SQUID. This is essential for operation of the device using conventional flux-locked loop readout electronics.
This approach has significantly advanced SQUID technology for numerous applications. However, unless the SQUID is heavily damped, the parasitic elements (i.e., capacitance and inductance) that are invariably introduced can lead to numerous resonances in the SQUID dynamics. These resonances manifest themselves as strong irregularities in the current-voltage (I-V) and in the voltage-flux (V-.PHI.) characteristics, leading to excess noise and making operation using conventional flux modulation techniques extremely difficult. Overdamping the SQUID may reduce the excess noise, but it also diminishes the amplitude of the SQUID output signal, placing more stringent demands on the readout electronics. To be useful for a wide variety of applications, a dc SQUID with transformer coupling of the input and modulation coils must meet several general requirements including: inductance matching of the input inductance to the load inductance, insensitivity to ambient fields, negligible coupling of the modulation signal to the input circuit, negligible coupling of the bias and modulation signals to the SQUID, and low-noise performance. To date, there have been few dc SQUID devices that adequately address all of these requirements.
Considerable effort has been devoted to the design of dc SQUIDs having low noise. Most approaches use a multi-layer design consisting of an input coil integrated on top of the SQUID inductance. In double washer designs wherein the washers are configured to form a gradiometer which rejects the effects of uniform fields, the bias current which must pass through the Josephson junctions may magnetically couple into the SQUID loop. This can result in an undesirable interaction that can introduce noise and drift into the SQUID sensor from the drive electronics. Further, introducing the bias current into the junctions in a non-symmetrical manner can also make the SQUID unduly sensitive to common mode noise which may be picked upon the bias leads that run from the electronics drive package at room temperature, down to the SQUID sensor in the cryogenic environment. Such common mode noise becomes an undesirable influence on the output signal.
In addressing these issues, U.S. Pat. No. 5,053,834 to Simmonds used a symmetrical dc SQUID system having two input coils and two modulation coils symmetrically arranged on a monolithic substrate such that the system nominally has no mutual inductance between groups of signal coils and modulation coils when the SQUID is biased for normal operation. The Simmonds SQUID response is gradiometric, and the Josephson junctions are in parallel with the input and modulation secondaries in order to keep the total SQUID inductance and flux noise low. The Simmonds configuration is designed to prevent bias current flowing into the junctions from coupling to the SQUID loop, to make the device more insensitive to fluctuations or noise in the bias current circuitry, and to prevent common mode noise on the bias leads, modulation coils or signal coils from coupling into the SQUID, but does not optimize device performance for applications which require detection of extremely weak currents. For such applications the equivalent rms current noise at the SQUID input is a much more relevant figure of merit than the rms flux noise, which is addressed by the Simmonds design. The present invention provides an improved dc SQUID design which incorporates the advantages provided by a symmetrical SQUID configuration while providing improved performance over prior art designs.